1. Field of the Invention
The present invention relates to a process for the preparation of polyester granules (compound) having improved photochemical and thermal stability for undyed or spun-dyed filaments, and the use thereof.
2. Prior Art
For reasons of environmental protection and for applications where long filaments and large amounts of filaments of constant levelness and good color fastness are required in thermoplastic filaments, the so-called spin dyeing process has successfully become established (DE-C-2 708 789). Predominantly polymer-soluble, heat-stable organic dyes having better or poorer lightfastnesses were used. With the constantly increasing requirements for lightfastness, for example in the automotive sector, where the dyed fiber material is exposed not only to the influence of light but simultaneously to the action of considerable heat, organic and inorganic pigments are increasingly being used. Since the color range of the pigments which can be used for spin dyeing is still very modest, the continued use of polymer-soluble dyes is unavoidable. Because these dye components are predominantly so-called shading dyes, the action of extreme light and heat results not only in the usual fading but in the even more serious deficiency of a color shift. To successfully protect the spun-dyed fiber material from UV radiation, so-called UV absorbers must additionally be used.
However, the technical application of these additives has proved particularly disadvantageous, particularly owing to their tendency to sublime and owing to their low melting point of less than 180.degree. C. Thus, conventional dusting methods, for example in the double-cone dryer or tumbler with simultaneous drying in high vacuum at temperatures above 150.degree. C., are ruled out owing to loss of additive and soiling of the evacuation units and caking of the granule/additive bed. Continuous metering of the additive into modern, integrated and more efficient spinning lines, i.e. addition directly before the extruder to the hot granules from the continuous dryer, is not feasible owing to the known problems, such as sintering, blockage of the granule pipe and bridging. Even the addition to the cold granules merely shifts the problems to the melting zone of the extruder, where feed problems and failure to control the pressure/speed regulation owing to premature melting of the additive make satisfactory and reliable production impossible.
It is also known that dye preparations can be injected directly to the polyester melt stream, shortly before the spinneret (Chemiefaser/Textil-Industrie, 1978, pages 1048/49); here, it is necessary to choose a carrier medium which must be liquid at the temperature of molten polyester and must be completely compatible with the latter.